This invention relates to a multilayer membrane and, more particularly, to a multilayer membrane, which consists of a carrier layer, a porous supporting layer and a separating layer. It also relates to process for manufacturing the membrane.
German Patent Application P 40 04 153.0-44 suggests a multilayer membrane which is composed of a carrier layer, a porous supporting layer and a separating layer. The carrier layer consists of fibers of polyamide, polyvinylidene difluoride, polyester and glass, the porous supporting layer consists of a polysulfone, polyimide, polyvinyl alcohol, polyurethane, polyvinylidene difluoride, polyether sulfone, polyacrylonitrile or polyether imide, and the separating layer consists of polyvinyl alcohol and is nonporous and has an ordered structure having a high microcrystalline content, in which the crystallites exhibit in the X-ray diffraction pattern a reflection in a range of 1.2 to 1.5 degrees at diffraction angles of from 19.6 to 20 degrees. German Patent Application P 40 04 153.0-44 also suggests the manufacture of this membrane by a process, in which a composite material consisting of the carrier layer and the porous layer is first made and an aqueous solution, which contains 1 to 10% by weight polyvinyl alcohol and has been stored at -10.degree. to 10.degree. C. for 3 to 20 days, is subsequently applied to the supporting layer of the composite material, and finally the composite material coated with the aqueous solution of polyvinyl alcohol is treated at 100.degree. to 180.degree. C. for one to 60 minutes.
The membrane suggested in German Patent Application P 40 04 153.0-44 has a high constant separating capacity, but further improvements in selectivity and separating capacity of the membrane were desired.